1. Field of the Invention
This invention relates generally to a treatment process for anode foils to be used in electrolytic capacitors. This invention also relates to a high capacitance anode foil capable of supporting voltages in excess of 750 Volts created by the foil treatment process, an electrolytic capacitor incorporating this anode foil, and an implantable Cardioverter Defibrillator (ICD) incorporating this electrolytic capacitor.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
An ICD system normally includes control electronics, a high current electrical battery cell, an energy storage reservoir (i.e., charge capacitor(s)), and a step-up transformer and power conversion circuitry to charge the capacitor(s). Typically, the ICD charges the charge capacitor(s) to a high voltage (700-800 Volts).
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Typically, these capacitors can be aluminum electrolytic capacitors (either rolled or flat).
Aluminum electrolytic capacitors having aluminum foil plates rolled into a very small volume are generally used in ICDs. However, flat, layered capacitors have recently been developed for use in ICDs. By etching the surface of the aluminum anode foil, the surface area can be further increased such that the capacitance increases accordingly.
Since these capacitors must typically store approximately 30-40 joules, their size can be relatively large, and it is difficult to package them in a small implantable device. Some patients who have a device implanted may be bothered by the presence of the large object in their pectoral region. Furthermore, the generally rectangular shape of some prior art devices can in some instances lead to pocket erosion at the somewhat curved corners of the device. For the comfort of the patient, it is desirable to be able to make smaller and more rounded ICDs. The size and configuration of the capacitors is a major factor in achieving this goal.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. As mentioned above, one way to increase capacitance per unit area in a flat capacitor is to etch the surface of the anode foil perpendicular to the surface thereof. An ICD with flat geometry electrolytic capacitors is described in U.S. Pat. No. 5,131,388 to Pless et al. ("the Pless patent"), which is incorporated herein by reference in its entirety. While such flat capacitors provide an improvement from a packaging and energy density standpoint, the energy or power density can still be greatly improved.
Conventionally, ICDs use two capacitors in series to achieve the desired high voltage for shock delivery. From the standpoint of size, it would be desirable to provide a capacitor arrangement for an ICD in a single package rather than two capacitors in series. However, this has not been possible since available anode foil technology has limited photo flash capacitor voltages to 400V or less.
It is important that the anode foil used in these capacitors maintains a high capacitance while at the same time has a reduced leakage current. The term "leakage current" refers to the current passing between an electrolyte and an anode foil. Under conventional anode foil preparation techniques, a barrier oxide layer is formed onto one or both surfaces of a metal foil by placing the foil into an electrolyte bath and applying a positive voltage to the metal foil and a negative voltage to the electrolyte. This formation process (also referred to as electrolysis) oxidizes the surface of the metal foil. The oxide film formed during formation normally has a thickness ranging from 0.006 to 1.0 micrometers (.mu.m). However, the oxide film must be sufficiently thick to support the intended use voltage. This oxide film acts as a dielectric layer for the capacitor, a barrier to the flow of current between the electrolyte and the metal foil, thereby providing a high resistance to leakage current passing between the anode and cathode foils. However, a small amount of current, the leakage current, still passes through the barrier oxide layer. A high leakage current can result in the poor performance and reliability of an electrolytic capacitor. In particular, a high leakage current results in greater amount of charge leaking out of the capacitor once it has been charged.
Various attempts have been made to reduce the leakage current properties of oxides formed on anode foils. For example, in a conventional anode foil formation process, such as described in U.S. Pat. No. 5,449,448 issued to Kurihara et aL (incorporated herein by reference), a hydration dip is utilized, where the aluminum foil is placed in a bath of heated pure water, followed by an organic acid "dip." Next, the barrier layer oxide is formed during electrolysis. The introduction of the organic acid dip into the formation process results in a reduced leakage current of the anode foil. However, the combination of the hydration dip and the organic acid dip also results in a reduced capacitance of the anode foil by as much as 7% or more.
During a hydration dip, a hydrated film or "hydrate layer" is formed on the surface of the aluminum foil as a result of the chemical reaction between the aluminum surface and the water at elevated temperatures. Briefly, during this chemical reaction, the oxygen in the water is stripped away from its hydrogen and bonds with the aluminum. As a result, the hydrogen is released and a crystalline aluminum oxide is formed on the surface of the aluminum. This crystalline structure is also referred to as Bohmite or pseudo-Bohmite.
On a microscopic level, the hydrate layer comprises two distinctly different layers, a dense layer (which is desired) and a more diffuse layer (which is not desired). The dense layer has a compact, crystalline structure. During electrolysis, this compact, crystalline oxide (i.e., dense) layer is incorporated in the barrier oxide layer in a smooth, homogeneous fashion such that when the barrier oxide layer is formed, it also has a compact or dense structure. Thus, the hydration step helps ensure that a desirable type of oxide is propagated during electrolysis.
In order to achieve the most optimum type of anode foil, the goal is to make the dense layer as thick as possible. Over time, the reaction rate of the aluminum with heated water falls exponentially so that after several hours, for example, the reaction rate falls to near zero. At this point in time, all of the available aluminum near the surface has chemically reacted with the hot water. However, by increasing the thickness of the hydrate layer, the thickness of an undesirable diffuse layer is also increased, while the thickness of the desirable dense layer remains largely unchanged.
The diffuse layer does not have a compact structure, though it is crystalline in nature. After a hydration step, as shown in FIG. 1A, diffuse layer 102 resembles acicular needles of aluminum oxide. Although the diffuse layer is also incorporated into the barrier layer, the result is a less dense and well ordered oxide with more interstitial vacancies and dislocations. In practice, after a formation step, and as depicted in FIG. 1B, after a dense layer 110 has reached its maximum thickness, there remains a substantial amount of the diffuse layer 112 projecting above barrier layer 111 that should be removed.
With respect to anode foil applications, the main problem with the unconverted diffuse layer is that it does not contribute much, if anything, to the amount of voltage an anode foil can withstand. Further, the diffuse layer negatively affects realizing ideal capacitance. By way of simple analogy, the diffuse layer acts to increase the distance between the anode (aluminum foil) and the cathode (the electrolyte), thereby decreasing the capacitance. It would be acceptable if the increase in distance also resulted in an increase in the amount of voltage that could be supported by the capacitor. But with a thick diffuse layer, there is little increase in the amount of voltage that the capacitor foil can support.
As mentioned above, some conventional foil treatment processes utilize the combination of the hydration dip followed by an organic acid dip in a foil treatment process. The organic acid dip serves to improve oxygen transport to both the dense and diffuse layers, resulting in a thicker hydrate layer for the same amount of time the foil is treated in a heated water bath. It is the ability to increase the dense layer thickness that is of primary importance. However, this combination of steps leads to a reduction in the capacitance of the foil. This unacceptable capacitance loss is likely due to excess thickness of the hydrate layer (i.e., the diffuse layer) that swells as a result of adsorbed organic acid from the organic acid dip. The excess diffuse layer formed as a result of the hydration step and organic acid dip is more easily dissolved than the true barrier oxide layer.
Manufacturers of electrolytic capacitors of very high capacitance, high voltage and small size have had great difficulty producing anode foils utilized in these capacitors that can support voltages of about 750 Volts or more. Conventional descriptions of commercial formation methods for electrolytic capacitors do not describe the formation of a high quality oxide with improved capacitance at voltages in excess of 750 volts. For example, U.S. Pat. No. 5,143,591 to Shaffer describes a method of producing ultra stable aluminum oxide for electrolytic capacitors. However, capacitors produced according to this method are capable of operating at voltages of less than 400 Volts. Thus, what is needed is a method of treating anode foils to produce a high quality oxide with reduced leakage current and improved capacitance at voltages exceeding 750 Volts.